The production of liquefied natural gas (LNG) is achieved by cooling and condensing a feed gas stream against multiple refrigerant streams provided by recirculating refrigeration systems. Cooling of the natural gas feed is accomplished by various cooling process cycles such as the well-known cascade cycle in which refrigeration is provided by three different refrigerant loops. One such cascade cycle uses methane, ethylene and propane cycles in sequence to produce refrigeration at three different temperature levels. Another well-known refrigeration cycle uses a propane pre-cooled, mixed refrigerant cycle in which a multicomponent refrigerant mixture generates refrigeration over a selected temperature range. The mixed refrigerant can contain hydrocarbons such as methane, ethane, propane, and other light hydrocarbons, and also may contain nitrogen. Versions of this efficient refrigeration system are used in many operating LNG plants around the world.
Another type of refrigeration process for natural gas liquefaction involves the use of a nitrogen expander cycle in which nitrogen gas is first compressed and cooled to ambient conditions with air or water cooling and then is further cooled by counter-current exchange with cold low-pressure nitrogen gas. The cooled nitrogen stream is then work expanded through a turbo-expander to produce a cold low pressure stream. The cold nitrogen gas is used to cool the natural gas feed and the high pressure nitrogen stream. The work produced by the nitrogen expansion can be used to drive a nitrogen booster compressor connected to the shaft of the expander. In this process, the cold expanded nitrogen is used to liquefy the natural gas and also to cool the compressed nitrogen gas in the same heat exchanger. The cooled pressurized nitrogen is further cooled in the work expansion step to provide the cold nitrogen refrigerant.
Refrigeration systems utilizing the expansion of nitrogen-containing refrigerant gas streams have been utilized for small liquefied natural gas (LNG) facilities typically used for peak shaving. Such systems are described in papers by K. Muller et al entitled "Natural Gas Liquefaction by an Expansion Turbine Mixture Cycle" in Chemical Economy & Engineering Review, Vol. 8, No. 10 (No. 99), October 1976 and "The Liquefaction of Natural Gas in the Refrigeration Cycle with Expansion Turbine" in Erdol und Kohie--Erdgas--Petrochemie Brennst-Chem Vol. 27, No. 7, 379-380 (July 1974). Another such system is described in an article entitled "SDG&E: Experience Pays Off for Peak Shaving Pioneer" in Cryogenics & Industrial Gases, September/October 1971, pp. 25-28.
U.S. Pat. No. 3,511,058 describes a LNG production system using a closed loop nitrogen refrigerator with a gas expander or reverse Brayton type cycle. In this process, liquid nitrogen is produced by means of a nitrogen refrigeration loop utilizing two turbo-expanders. The liquid nitrogen produced is further cooled by a dense fluid expander. The natural gas undergoes final cooling by boiling the liquid nitrogen produced from the nitrogen liquefier. Initial cooling of the natural gas is provided by a portion of the cold gaseous nitrogen discharged from the warmer of the two expanders in order to better match cooling curves in the warm end of the heat exchanger. This process is applicable to natural gas streams at sub-critical pressures since the gas is liquefied in a free-draining condenser attached to a phase separator drum.
U.S. Pat. No. 5,768,912 (equivalent to International Patent Publication WO 95/27179) discloses a natural gas liquefaction process which uses nitrogen in a closed loop Brayton type refrigeration cycle. The feed and the high pressure nitrogen can be pre-cooled using a small conventional refrigeration package employing propane, freon, or ammonia absorption cycles. This pre-cooling refrigeration system utilizes about 4% of total power consumed by the nitrogen refrigeration system. The natural gas is then liquefied and sub-cooled to -149.degree. C. using a reverse Brayton or turbo-expander cycle employing two or three expanders arranged in series relative to the cooling natural gas.
A mixed refrigerant system for natural gas liquefaction is described in International Patent Publication WO 96/11370 in which the mixed refrigerant is compressed, partially condensed by an external cooling fluid, and separated into liquid and vapor phases. The resulting vapor is work expanded to provide refrigeration to the cold end of the process and the liquid is sub-cooled and vaporized to provide additional refrigeration.
International Patent Publication WO 97/13109 discloses a discloses a natural gas liquefaction process which uses nitrogen in a closed loop reverse Brayton-type refrigeration cycle. The natural gas at supercritical pressure is cooled against the nitrogen refrigerant, expanded isentropically, and stripped in a fractionating column to remove light components.
The liquefaction of natural gas is very energy-intensive. Improved efficiency of gas liquefaction processes is highly desirable and is the prime objective of new cycles being developed in the gas liquefaction art. The objective of the present invention, as described below and defined by the claims which follow, is to improve liquefaction efficiency by providing two integrated refrigeration systems wherein one of the systems utilizes one or more vaporizing refrigerant cycles to provide refrigeration down to about -100.degree. C. and utilizes a gas expander cycle to provide refrigeration below about -100.degree. C. Various embodiments are described for the application of this improved refrigeration system which enhance the improvements to liquefaction efficiency.